Recently, the epitaxy technique is getting more and more improved, so it is very possible to have a double heterostructure with excellent quality, and which may provide LED more than 90% quantum efficiency. However, emitting layer of a typical LED is formed by epitaxially growing; therefore, the growing speed is very slow.
Low-dimensional structures including nanoparticles or quantum dots (QDs) are supposed to provide significant enhancement in the density of states, so it increases the probability of light emission. Those low-dimensional structures can be epitaxially grown on bulk materials like GaAs wafers or separately formed by chemical methods.
Luminescent nanoparticles formed by chemical methods have many advantages. First, it can be dissolved in the solvent to become a solution. Second, it can be applied on any substrates by any process such as spray, dip coating, or spin coating. Third, the speed can be very fast (several micrometers per second); therefore, area or volume density of the material can be very high.
The expitaxially growing way is very selective on the grown substrates. Also, QDs are usually formed with only a scarce area density. Thus, the forming speed is very slow (such as several micrometers per hour). Furthermore, it also needs expensive vacuum equipment to carry out the necessary process. The process to fabricate monodispersed nanoparticles is inexpensive and facile for industrial application. Stimulated emission and optical gain had been demonstrated in CdS quantum dots by optical pumping methods. This encourages the employment of electrical pumping to realize efficient nanoparticle-based light emitting devices.